Effects of Litter Size and Parity on Farrowing Duration of Landrace × Yorkshire Sows

Simple Summary Litter size is an important economic trait in pigs. Improving the number born alive is an important breeding goal of the pig husbandry. A shorter farrowing duration is welcome for facilitating the management and sows’ health. Therefore, the aim of this study was to explore the effect of litter size and parity on farrowing duration, to determine whether a shorter length of farrowing duration could be considered as a breeding parameter in pig breeding. Our results showed the total number born had no significant relation with farrowing duration, but number of stillbirths increased with the prolongation of farrowing duration and decrease of live litter size if farrowing duration was longer than 240–300 min. Different parities sows had little difference in the same farrowing duration interval except for gilts. A shorter farrowing duration within 300 min might be considered in pig breeding without worrying about the decreasing of live litter size or the negative effect of parity. Abstract Litter size has increased and farrowing duration has also prolonged in recent years. The aim of this study was to analyze the effect of litter size and parity on farrowing duration (FAR) to estimate the possibility of selecting a short farrowing duration. We recorded 32,200 parturitions of 8420 Landrace × Yorkshire sows, determined farrowing duration, litter size, parity, gestation length. Results showed that total number of born (TNB) and parity obeyed a cubic (p = 0.0004, p = 0.004) relationship while number born alive (NBA) and number born dead (NBD) obeyed a linear (p = 0.0239, p = 0.0035) relationship with FAR. Gestation length obeyed a linear (p = 0.02) relationship with FAR. FAR of sows with stillbirth was longer than that of sows without stillbirth. Stillbirth rate increased rapidly from about 2% to 4%, especially when FAR was over 240 min. FAR gradually prolonged with the parities. FAR of 7th parity sows was longer than that of 1st~6th parity sows (p < 0.05), but different parity sows had little difference in the same FAR interval except for gilts. Results indicated it was possible and necessary to consider FAR into pig breeding without worrying about decreasing of live litter size or negative effect of parity if FAR was shorter than 300 min.


Introduction
Reproductive traits are the most important concepts in determining the output of a pig farms. Litter size is an important reproductive trait, and it is one of the decisive factors affecting the economic benefits of the entire pig industry. Over tens of reproduction traits are considered in calculating the breeding index to rank and select breed pigs. However, the average stillbirth rate of piglets was still 3-8%, of which approximately 75% happened during parturition [1], the increase of stillbirth rate would inevitably reduce the profits Animals 2022, 12, 94 2 of 12 of pork production [2,3]. Larger litter size may lead to a longer farrowing duration (FAR) and is related to the lower birth weight of piglets [4]. It has been suggested that large litter size can also lead to the increase of stillbirth rate, which is likely to bring significant negative impact on animal welfare [5]. Farrowing is known to be a stressful process of both piglets and sows, and a longer duration of farrowing process often requires extra care from nursing staff and sometimes impairs the uterus health of sows, which would bring certain difficulties in management and even a fertility reduction of sows during the next parturition. Many stillbirths occur during parturition due to dystocia and prolong farrowing.
Duration of farrowing (FAR) was reported to prolong with the number of stillbirths [6]. A prolonged FAR is said to impair fertility in pigs [7], as sows with a longer FAR tended to have a higher risk of post parturient disorders [8]. However, at the same time, the prolongation of FAR was also in relation to the increase of litter size [9,10]. In addition, genetic, breed, age, length of gestation, feeding management, environment and nutritional factors could also affect FAR [11]. There have been some studies focused on the environment conditions control in the farrowing house in order to ensure the smooth production of piglets [12][13][14].
The aim of this study was to explore the exact effect of litter size including total number born, number born alive and number dead on FAR. We also wanted to evaluate the impact of parity on FAR and litter size in order to evaluate the possibility of short FAR as a breeding indicator for pigs.

Animals
The observational study was conducted in 2017-2018 on a commercial pig farm in Central China. Parturitions (n = 32,231) of 8420 randomly selected healthy Landrace × Yorkshire hybrid sows between 1st and 10th parity were included in this study. The sows farrowed in 26 batches. Pregnant sows were fed 1.8-2.0 kg/day for the first 7 days; 2.0-2.5 kg/day from days 7 to 30; 2.5-3.0 kg/day from days 30 to 70; 2.5 kg/day from days 70 to 90; and 3.5 kg/day from days 90 to 110. The diets were formulated according to their body condition. During 3 days before farrowing, sows were fed 2.5 kg/day. On the day of farrowing, sows were fed 0.5-1.5 kg/day. For the first 5 days after parturition, sows were fed 2.0, 2.5, 3.0, 3.5, and 4.0 kg/day, respectively.
All pregnant sows were fed in groups and transferred to farrowing houses 1 week before farrowing and fed in single crates in farrowing houses with free access to drinking water. The farrowing pens were 2.50 × 2.30 m 2 in size, the farrowing crates were 2.10 × 0.70 m 2 in size. The average room temperature was 25 • C. The environment of this pig farm was controlled by Big Dutchman ventilation system and cooling pad in summer and warm air heating system in winter. All sows were raised under regular disinfection and vaccination procedures (Table 1).

Measurements
Duration of farrowing (FAR) was defined as the time interval between the first birth to the complete expulsion of placenta. We also determined the following sow traits: farrowing batch, farrowing barn, farrowing house, farrowing pen, the length of pregnancy, parity, feeders and nursing staffs. The length of pregnancy was defined as the time interval between fertilization and farrowing. Total number born piglets (TNB), number of born alive piglets (NBA) and number of born dead piglets (NBD) in each litter were investigated by nursing staffs. TNB consisted of NBA, NBD and mummies. Piglets that had no breathing and heartbeat at birth, or had heartbeat but no breathing and unable to breathe after rescue, were defined as NBD.
FAR and delivery mode (normal delivery and assisted delivery) were recorded by the nursing staff who works day and night shifts in farrowing houses. Nursing staffs' job is for delivery and nursing. When the sow's amniotic fluid is out, they need to wipe the sow's udder and hind body with warm water and disinfect with 0.1% potassium permanganate solution and wait for the sow to farrow. They need to keep an eye on each sow in farrowing and keep an eye on each sow as she gives birth, in order to deal with the dystocia of sows, the suspended death of newborn piglets, and to cut umbilical cord, to assist the piglets to keep warm and access the colostrum as soon as possible.
The assisted delivery was adopted if sows had strong parturition symptoms such as giving an expulsion effort but no piglet was delivered after 1-2 h of amniotic fluid outflow, or a fetus was present in canal or uterine cervix but sows had not enough labor force or the litter interval was more than 1 h. The assisted delivery consists of an injection of 0.5-1.0 mL oxytocin or and an artificial assisted delivery.

Statistical Analysis
All the data was analyzed by SPSS Statistics 26 (IBM SPSS Statistics, IBM Corp, Armonk, NY, USA) and presented with R software. Variables including FAR, TNB, NBA, NBD, parity (P) and gestation length (G) were checked for normal distribution. An initial univariable screening was performed to identify potential influencing factors of FAR and litter size. The correlations of FAR and TNB, NBA, NBD, P and G were tested and estimated. The regression model that had the highest fitting degree and achieved significance level was selected. Based on the significance levels of those tests, a multivariable linear model for FAR was created. Farrowing house, farrowing pen, nursing staff, feeder, and batch were included as random factors and delivery mode as fixed effects. The correlations of parity (P) and gestation length (G) on litter size were tested and estimated. Based on the significance levels of those tests, a multivariable linear model for TNB and NBA and a lineal model for NBD were created. Farrowing house, farrowing pen, nursing staff, feeder, and batch were included as random factors and delivery mode as a fixed effect. Results were presented as Mean ± SD and considered significant at p ≤ 0.05.

Results
Parturition records of FAR longer than 10 h (n = 225), live litter size less than 5 (n = 1172) and more than 19 (n = 31) were excluded. Data of parity ≥ 7 were classified as parity 7. A total of 32,200 parturitions of 8420 Landrace × Yorkshire hybrid sows were included in the following analysis: 3451 assistant delivery parturitions (10.7%). Distribution of farrowing duration, litter size, parity and gestation length were presented in Figure S1.

Effects of Litter Size on Farrowing Duration
Total number of born (TNB) (5-20), number born alive (NBA) (5~18) and number born dead (NBD) (0-9) obeyed a cubic (R 2 = 0.77, p = 0.0004; R 2 = 0.54, p = 0.0442; R 2 = 0.79, p = 0.0043) relationship with farrowing duration (FAR) (10.2-600 min) ( Figure 1). In the final multivariable model for FAR, the interact of TNB and NBA (TNB × NBA) were significant (p = 0.012). The average FAR was 250.05 ± 67.63 min, TNB was 11.61 ± 2.45, NBA was 11.29 ± 2.42, and NBD was 0.26 ± 0.58, respectively. An overview of effect of litter size on FAR was listed in Figure 1 and Tables 2-4. final multivariable model for FAR, the interact of TNB and NBA (TNB × NBA) were significant (p = 0.012). The average FAR was 250.05 ± 67.63 min, TNB was 11.61 ± 2.45, NBA was 11.29 ± 2.42, and NBD was 0.26 ± 0.58, respectively. An overview of effect of litter size on FAR was listed in Figure 1 and Tables 2-4. From Table 2, farrowing duration indicated no significant difference between different TNB, but significant difference between different NBA (p < 0.05). Sows of different TNB and NBA had almost the same FAR variations, about 27%. The length of FAR first decreased and then increased with the increase of NBA. When litter size was less than 11, NBA took a little longer FAR than TNB. When litter size was over 12, NBA generally took a shorter time to farrow than TNB. Sows with 5 and 16 NBA had the longest FAR of 257.49 ± 76.8 min and 256.62 ± 67.61 min, longer than sows with 13, 17 and 18 NBA. Sows with 18 NBA had the shortest FAR of 239.98 ± 65.88 min, which was shorter than that of all the other sows, followed by sows with 17 and 13 NBA of 246.21 ± 65.05 min and 247.19 ± 65.89 min. Table 2. Descriptive statistics of farrowing duration of different total number of born and number born alive piglets.  From Table 2, farrowing duration indicated no significant difference between different TNB, but significant difference between different NBA (p < 0.05). Sows of different TNB and NBA had almost the same FAR variations, about 27%. The length of FAR first decreased and then increased with the increase of NBA. When litter size was less than 11, NBA took a little longer FAR than TNB. When litter size was over 12, NBA generally took a shorter time to farrow than TNB. Sows with 5 and 16 NBA had the longest FAR of 257.49 ± 76.8 min and 256.62 ± 67.61 min, longer than sows with 13, 17 and 18 NBA. Sows with 18 NBA had the shortest FAR of 239.98 ± 65.88 min, which was shorter than that of all the other sows, followed by sows with 17 and 13 NBA of 246.21 ± 65.05 min and 247.19 ± 65.89 min.
From Table 3 From Table 4, after dividing FAR into 5 groups: 0-180 min, >180-240 min, >240-300 min, >300-360 min and >360-600 min, number born dead and stillbirths rate increased with the length of FAR, the longer the FAR, the more the NBD (from 0.21 to 0.45) and stillbirth rate (from 1.83% to 0.45%), especially when FAR was longer than 240 min. When the length of FAR was over 240 min, there would be significantly more NBD (p < 0.05).

Effect Gestation Length on Farrowing Duration and Litter Size
Gestation length (109-118 days) obeyed a lineal (R 2 = 0.51) relationship with FAR, TNB (R 2 = 0.946, p = 0.0004) and NBA (R 2 = 0.915, p = 0002) (Figure 3). No significant relationship was found between gestation length and NBD. The average gestation length (G) was 114.28 ± 0.98 days. FAR was positively associated with length of gestation (β = 3.67; p = 0.02). An overview was shown in Table 7. There were only seven and three parturitions of gestation length of 109 days and 118 days. Additionally, significant difference was only found among sows of gestation length of 109 days and 118 days for FAR, TNB and NBA. After removing these 10 parturitions records, there was no difference in FAR between 110-117 days of gestation length. TNB and NBA of sows with 112 and 113 days of gestation were significantly higher than those in other gestations.
After removing these 10 parturitions records, there was no difference in FAR between 110-117 days of gestation length. TNB and NBA of sows with 112 and 113 days of gestation were significantly higher than those in other gestations.

Discussion
A longer duration of farrowing (FAR) is thought to be against the health of sows [15] and their next delivery performance which would reduce the working efficiency for feeders and nursing staffs in farrowing houses. Some studies also suggested a positive correlation between FAR and litter size, the longer FAR, the more litter size. Therefore, in this study, we investigated the influence of litter size including TNB, NBA and NBD on FAR and the impact of parity on both FAR and litter size using a large scale of farrowing data in order to look into whether it was reasonable to consider a shorter FAR as one of the breeding parameters in pigs in the further.
By analyzing 32,200 parturition records from 8420 Landrace × Yorkshire sows, we found that the average length of FAR was about 250 min, or about 4.2 h, which was another piece of evidence that the FAR is getting longer and longer. In 2004, FAR was reported to last 133 min [9], and increased to 166 min [6] and 268 min [12], though the common use of oxytocin-like compound in pig production can shorten the length of FAR [16]. This might be partly because of the rapid increase of litter size in pigs since 2004. The average length of FAR in this study was significantly shorter than the 6~8 h of a batch of European superior sows [17]. Average TNB in this study was 11 compared to as high as 20 in the most hyper prolific sows in Europe.
In this study, we found that the litter size and parity had an impact on FAR by using the multivariable linear model. The interaction between TNB and NBA were detected. In the study of Bjorkman et al., they used the multinomial logistic regression model to explore the relationship between litter size, parity and farrowing duration. They found a positive correlation between number born alive and FAR, and number of stillbirth was significantly correlated with FAR [18]. The coefficient of variation of TNB and NBA were larger, about 27%. Significant difference was not found in FAR between different TNB, but found between different NBA because of the interact between FAR and stillbirth [19]. Shortening the length of FAR will not reduce the effective NBA. FAR of >240 to 300 min might be the ideal cut-off point in this population (with 5 to 22 litter size). This was in coincidence with the general believing that FAR that exceeded 300 min was likely to cause dystocia and new-born piglet complications, and piglets that experienced a longer FAR are more likely to die at birth [17,20].
Besides, NBD increased gradually with FAR, especially when FAR was longer than 240 min, the NBD increased rapidly and linearly. Studies have shown that a longer FAR could affect perinatal mortality or subsequent piglet growth. FAR and the birth order of piglets were reported to be the main factors determining the risk of stillbirth [20]. Some studies also pointed that there was a positive correlation between the number of born alive and FAR, and the number of stillbirths was significantly negatively correlated with FAR [18,21].
In addition, we found that parity had a significant impact on FAR. As the parity increased, FAR gradually became longer. Studies show that parity has a slight effect on the number of colostrum metabolites, which might affect the reproduction performance of sows, including litter weight at birth and piglet mortality [22]. Parity and FAR could affect the incidence of postpartum disease in sows; if FAR was longer than 4 h, the sows would be at greater risk for fever 1 day after parturition [23], which affected the reproductive performance of sows and sped up the sows' elimination. Besides, the effect of parity on FAR might be due to the aging of sows' uteri after multiple parturitions, which weakens their muscles' ability to contract during parturitions, resulting in prolonged FAR.
Some studies have shown that FAR was not only affected by parity, but also by the sow's physical condition, feeding and environment. For example, a higher energy intake in sows during late gestation could improve the farrowing duration traits [24], adding the Bacillus during the perinatal period of sows could shorten FAR and the weaning-estrous interval [25]. FAR was positively correlated with the gestation, and FAR with 118 day of pregnancy was the longest. However, there was no significant difference between gestation (110-117 days) and FAR after removing the data of pregnancy was 109 and 118. A study reported that the length of gestation (112-119 days) was negatively correlated with FAR, but the effect of litter size on FAR was not considered [6].
In general, we found that litter size had an impact on FAR, and there was a positive correlation between parity and FAR. The total number born and the number born alive increased gradually in 1-4th parity. All of these indicated that we can select sows with a shorter FAR and of the 1-4th parity during pig breeding.

Conclusions
In summary, we found that number of stillbirths increased with the increase of farrowing duration and decrease live litter size if farrowing duration was longer than 240-300 min. It might be possible to choose a shorter farrowing duration without compromising decreasing litter size in pig breeding. In order to further confirm the effect of litter size and parity on farrowing duration, it is necessary to further explore it from the perspective of genome.